It is generally known that particle size distribution, morphology and agglomeration of the titanium dioxide particles strongly influence the properties of the obtained pigments.
U.S. Pat. No. 4,241,042 discloses a method of preparing titanium dioxide, having the above-mentioned characteristics, via aerosol, which comprises preparing a liquid aerosol of a hydrolizable Ti(IV) compound, and reacting, in a subsequent step, the resulting aerosol with water vapor in order to obtain hydrated dioxide which, after drying at a temperature of 120.degree.-130.degree. C., is converted to the crystalline phase anatase; if subjected to higher temperatures, of the order of 1100.degree. C., it is converted to rutile.
This method permits to obtain a titanium dioxide with good properties, particularly if a nucleating agent such as AgCl is used.
An alternative method, always via aerosol, which permits to avoid the mixing step, is described in U.S. Pat. No. 4,574,078.
These methods lead to good results, but involve of course the formation of an aerosol which, as is well known, requires exactly controlled conditions to provide a titanium dioxide having the above-described characteristics.
A method of preparing a titanium dioxide having the aforesaid characteristics through an alternative process, i.e. homogeneous precipitation, is described in "Preparation and Mechanism of Formation of Titanium Dioxide Hydrosols of Narrow Size Distribution", E. Matijevic, M. Budnik and L. Meites in "Journal of Colloid and Interface Science" Vol. 61, (2), 1977.
The process consists in subjecting to hydrolysis at a temperature of about 100.degree. C., TiCl.sub.4 solutions which are acid due to HCl, in the presence of sulphate ions.
Said process, however is affected by the drawback of a very low productivity, as it requires very long times, of the order of a few days.
Thus, a commercial-scale process cannot be carried out on the basis of said operative technique.
There was the strong requirement of having available a homogeneous precipitation technique, which should permit to obtain a titanium dioxide with the abovesaid characteristics and with a high productivity.